Radio communication system, radio communication method, radio base station, and radio terminal station

ABSTRACT

There is disclosed a multicast transmission system in which an efficient and highly reliable multicast transmission can be performed. In the multicast transmission system for performing multicast transmission to a plurality of terminal stations from a base station, when an error is detected in the terminal station, the terminal station utilizes some of sub-carriers constituting an OFDM symbol to generate and transmit a NAK signal to the base station. A level judgment section  25  in the base station resends a packet to each terminal station when a reception signal level exceeds a threshold. Since the number M of sub-carriers able to be utilized to generate the NAK signal and the number L of sub-carriers necessary for generating the NAK signal are determined base on the number of terminal stations, packet communication quality, and the like, an erroneous detection probability and detection miss probability of the NAK signal can both be lowered.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The subject application is related to subject matter disclosed inJapanese Patent Application No. H11-275225 filed on Sep. 28, 1999 inJapan to which the subject application claims priority under ParisConvention and which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a data transmission method inradio multicast communication. Particularly, the present inventionrelates to a radio multicast communication in which when an error isdetected in a multicast-transmitted packet, a negative acknowledgment(hereinafter referred to as NAK) is returned as a response to a basestation to thereby perform a request for resending.

[0004] 2. Related Background Art

[0005] When performing multicast communication in a radio communicationsystem, there is an advantage that all terminal stations able tocommunicate with a base station can transmit/receive information and theinformation can be transmitted to all the terminal stations at onetransmission. However, on the other hand, when performing a request forresending because of an error generated in a transmission line, if aplurality of terminal stations simultaneously perform requests forresending, there is a problem that the requests collide against oneanother on a radio circuit and resending request (NAK) information isnot correctly transmitted.

[0006] The problem about signal collision caused when the plurality ofterminal stations utilize the same circuit is well known as amulti-access problem, and various solution methods have been proposed.

[0007] For example, a transmission right control system of issuingsignal transmission rights for transmission acknowledgment (JapanesePatent Application Laid-Open No. 46161/1999), a system of transmitting aNAK signal provided with a packet number not normally received by randomaccess when an error is generated in a reception signal (Japanese PatentApplication Laid-Open No. 210031/1998), and a system of transmitting aburst signal as the NAK signal to a time position corresponding to thepacket number when the error of the reception signal is detected(Japanese Patent Application Laid-Open No. 53089/1993) are known.

[0008] In the first transmission right control system,transmission/reception of information for adjusting a returning timingof a resending request signal is necessary, and there is a problem thatcontrol is complicated. Moreover, in a mobile communication system inwhich the terminal station moves, since the terminal station as amulticast communication target changes, the control is furthercomplicated.

[0009] In the second random access system, the resending request signalof the multicast communication is frequently generated in a plurality ofterminal stations at the same time, a probability of occurrence ofcollision of the NAK signals is high and an efficiency is deteriorated.To reduce the collision, a back off time needs to be taken before thetransmission of the NAK signal. However, when the number of multicastaddress terminal stations increases, the back off time needs to beincreased, and efficiency deterioration attributed to the back off timecannot be ignored.

[0010] In the third burst signal system, similarly as the random accesssystem, the probability of occurrence of collision is high. However, todetect a signal energy in the time position, even when the NAK signalsfrom a plurality of terminal stations collide with one another, somesignal energy is detected, and it can therefore be recognized that thecorresponding packet is erroneously received by at least one terminalstation. In this system, however, a signal energy detection precisionraises a problem. For example, when two signals subjected to PSKmodulation are received by a multipath with a phase deviating by 180degrees, the signal energy becomes zero, and the base station as themulticast transmission station cannot detect that an error is generatedin a reception station for receiving the packet.

[0011] Moreover, in the present system, since an erroneous packet isspecified by the time position for transmitting the burst signal, with adetection miss (although the burst signal is received, it is judged thatthere is no burst signal) resending of the erroneous packet is notperformed. In order to reduce the detection miss, when a threshold fordetection is lowered, erroneous detection (although no burst signal isreceived, it is erroneously judged that there is a burst signal) iseasily caused by influence of disturbances such as an undesirable noise,and unnecessary resending is performed.

[0012] That is, various solution methods of multiple access have beenproposed, but there are problems such as a complicated control and aninsufficient effect.

[0013] Incidentally, because of completion of IEEE 802.11 radio LANstandards in 1997, and advancement in price reduction of radio LAN, alarge number of radio LAN products have been placed on the market.

[0014] At present, aiming at a higher speed of the radio LAN, in IEEE802.11 committee, specifications of the radio LAN using a radiofrequency of 5 GHz band are studied, and it is determined that anorthogonal frequency division multiplexing (OFDM) system strong againstmultipath interference is used as a transmission system.

[0015] On the other hand, in a current IEEE802.11 resending controlmethod, when performing unicast transmission for transmittinginformation to one specific terminal station, if the transmitted packetis correctly received, the terminal station returns an acknowledgmentsignal (hereinafter referred to as the ACK signal) after a time intervalcalled a short interframe space (SIFS).

[0016] However, for the multicast communication including the multicasttransmission, no acknowledgment is made in the specifications.Specifically, since the resending control in a radio link is notapplied, reliability of information transmission is low in the multicasttransmission, and further there is a problem that the data transmissionefficiency is lowered by the resending control of an upper layer.

SUMMARY OF THE INVENTION

[0017] The present invention has been developed in consideration of theaforementioned problems, and an object thereof is to provide a multicasttransmission system in which an efficient and highly reliable multicasttransmission can be performed.

[0018] To achieve the aforementioned object, there is provided a radiocommunication system for performing transmission/reception of a packetin a multicarrier transmission system between a base station and aplurality of terminal stations,

[0019] wherein each of the plurality of terminal stations comprises:

[0020] a receiver for receiving a multicast transmission packettransmitted to each of the terminal stations from the base station;

[0021] an error detector for detecting whether or not there is an errorin the multicast transmission packet received by the receiver;

[0022] sub-carrier selector for selecting L (M≧L, L is an integer)pieces of sub-carriers from at least M (M≧1, M is an integer) pieces ofsub-carriers included in a resending request signal to the multicasttransmission packet; and

[0023] terminal station transmitter for transmitting a signal obtainedby superposing a modulation signal only to the selected L pieces ofsub-carriers as the resending request signal to the base station, and

[0024] the base station comprises:

[0025] a judgment section for judging, based on the resending requestsignal received from the plurality of terminal stations, whether or notthe previously transmitted multicast transmission packet is resent; and

[0026] a resending section for resending the multicast transmissionpacket to the plurality of terminal stations when the judgment sectionjudges that the multicast transmission packet is to be resent.

[0027] According to the present invention, since the resending requestsignal is generated using only some of the sub-carriers constituting areception packet OFDM symbol, an erroneous detection probability anddetection miss probability of the resending request signal can bereduced, and a highly reliable multicast transmission is possible.

[0028] Moreover, in the present invention, since OFDM transmissionutilizes easy realization of orthogonality in a frequency axis, thesystem has a priority of realizing properties to the similar systemutilizing the orthogonality in a time axis.

[0029] Furthermore, since the present invention can be applied not onlyto a concentrated control type radio system in which the base stationperforms allotment of a radio band but also to a random access radiosystem on the basis of CSMA, the present invention can also be appliedto the existing IEEE 802.11 radio LAN system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a block diagram showing a constitution of a terminalstation of a first embodiment.

[0031]FIG. 2 is an explanatory view of a multicast transmissionprinciple.

[0032]FIG. 3 is a view showing one example of NAK signals transmitted byrespective terminal stations.

[0033]FIG. 4 is a view showing one example of the NAK signals receivedby a base station.

[0034]FIG. 5 is a chart showing a size relation between L and M.

[0035]FIG. 6 is a block diagram showing the constitution of the terminalstation when the terminal station determines a value of L.

[0036]FIG. 7 is a block diagram showing a constitution of the basestation of the first embodiment in which multicast transmission isperformed to the terminal stations shown in FIG. 1 and 6.

[0037]FIG. 8 is a block diagram showing an internal constitution of alevel judgment section of FIG. 7.

[0038]FIG. 9 is a block diagram showing a constitution of the basestation of a second embodiment.

[0039]FIG. 10 is a block diagram showing a constitution of the basestation of a third embodiment.

[0040]FIG. 11 is a chart showing a transmission procedure of multicasttransmission in the aforementioned first to third embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] A multicast transmission system according to the presentinvention will concretely be described hereinafter with reference to thedrawings.

[0042] (First Embodiment)

[0043] In the multicast transmission system of the present invention, abase station simultaneously performs a multicast transmission of apacket to a plurality of terminal stations.

[0044]FIG. 1 is a block diagram showing a constitution of the terminalstation of a first embodiment, and FIG. 2 is a view of explainingprinciple of a multicast transmission. Before describing theconstitution of FIG. 1, an outline of the multicast transmission will bedescribed with reference to FIG. 2.

[0045] The base station performs the multicast transmission of thepacket simultaneously to a plurality of terminal stations (t1 to t2 ofFIG. 2). Each terminal station detects an error of a reception packet byusing CRC check and the like. As a result, when an error is detected inthe reception packet, a NAK signal is generated.

[0046] The NAK signal is constituted of one OFDM symbol. Usually, theOFDM symbol is generated by superposing a modulation signal to Nsub-carriers crossing at right angles to one another and performing aninverse Fourier transform (IFFT) processing, but in the presentembodiment, one sub-carrier is utilized to generate the NAK signal (OFDMsymbol). Additionally, a technique of generating the NAK signal will bedescribed later in detail.

[0047] Since a time slot for the base station to perform the multicasttransmission of the packet (t1 to t2 of FIG. 2), and a time slot for theterminal station to return the NAK signal to the packet (t2 to t3 ofFIG. 2) are predetermined by advance procedure, each terminal stationuses a designated time slot (t2 to t3 of FIG. 2) to transmit the NAKsignal to the base station.

[0048] The first embodiment of the terminal station shown in FIG. 1 willnext be described. The terminal station of FIG. 1, as a reception systemconstitution, is provided with an RF section 2 for down-converting aradio frequency signal received by an antenna 1 to perform orthogonaldemodulation, an OFDM symbol detector 3 for performing FFT processing toan output of the RF section 2 to detect the OFDM symbol, a P/S converter4 for performing a parallel/serial conversion of the OFDM symbol, ademodulator 5 for demodulating the serially converted OFDM symbol, anencoder (error detector) 6 for using the CRC check or the like to detectan error of a demodulation signal, a sub-carrier selection section(sub-carrier selector) 7 for selecting some sub-carriers when the erroris detected, and a controller 8 for using the selected sub-carriers togenerate the NAK signal.

[0049] The sub-carrier selection section 7 selects some (L) sub-carriersconstituting the OFDM symbol. As a method of selecting the sub-carrierin the sub-carrier selection section 7, any method may be selected froma method of selecting the sub-carriers at random every time, a method ofselecting the sub-carriers at random only at communication start andsubsequently selecting the same sub-carriers, a method of selectingfixed sub-carriers, and the like.

[0050] The sub-carrier selection section 7 notifies the controller 8 ofthe selected sub-carriers. The controller 8 superposes the modulationsignal only to the selected L sub-carriers, and generates a signalseries such that other sub-carriers are null.

[0051] Moreover, the terminal station of FIG. 1, as a transmissionsystem (terminal station transmitter) constitution, is provided with anencoder 9 for encoding a transmission signal to generate a signalseries, a multiplexer 10 for multiplexing the respective signal seriesgenerated by the encoder 9 and controller 8, a modulator 11 formodulating a multiplexed signal, an S/P converter 12 for converting themodulation signal to a parallel signal, an OFDM symbol generator 13 forperforming an IFFT processing to an output of the S/P converter 12 togenerate the OFDM symbol, and an RF section 14 for modulating the OFDMsymbol for up-conversion to a radio frequency, and an output of the RFsection 14 is transmitted via the antenna 1.

[0052] The multiplexer 10 outputs the signal series generated by theencoder 9 when the controller 8 generates no NAK signal, and multiplexesthe signal series generated by the encoder 9 with the signal seriescorresponding to the NAK signal when the controller 8 generates the NAKsignal.

[0053] Additionally, the drawings show only a minimum constitution todescribe the present invention, but for example, to perform interleaveor forward error correction (FEC), an interleaver immediately after theencoder 9, a deinterleaver immediately before the encoder 6, and thelike are necessary.

[0054]FIG. 3 is a chart showing one example of the NAK signalstransmitted by the respective terminal stations, and shows an example inwhich when the total number of sub-carriers is N, and the number of thesub-carriers of the NAK signals is 1, only sub-carrier sub3 is used totransmit the NAK signal.

[0055]FIG. 4 is a chart showing one example of the NAK signals receivedby the base station. Each of diagonal line parts of FIG. 4 shows the NAKsignal.

[0056] As shown in FIG. 4, when the sub-carriers of the NAK signalstransmitted by the respective terminal stations are different from oneanother, a reception level of each sub-carrier fails to be lowered.

[0057] The present embodiment is characterized in that even whencollision of the NAK signals occurs, the reception level of eachsub-carrier fails to increase or decrease.

[0058] Therefore, in the present embodiment, during generation of theNAK signals by the respective terminal stations, a probability ofselecting the same sub-carrier is set to be as small as possible. Inorder to minimize this probability, it is most preferable to set thenumber L of sub-carriers necessary for generating the NAK signal to 1,and set the number M of sub-carriers usable for generating the NAKsignal to N (N is the total number of sub-carriers constituting the OFDMsymbol).

[0059] However, when L is set to 1, M is set to N, communication qualityis satisfactory, and no NAK signal is returned from the terminalstation, an erroneous detection probability that although no NAK signalis present, the presence is erroneously judged increases. This isbecause the erroneous detection probability increases in proportion to avalue of M. Therefore, from a viewpoint of the erroneous detectionprobability, M is preferably set to be as small as possible.

[0060] On the other hand, from a viewpoint of a detection missprobability judged that although the NAK signal is present, the NAKsignal is not present, L is preferably set to be as large as possible.However, the larger M is and the smaller L is, the larger a possibilityof selecting the same sub-carrier becomes.

[0061] A size relation between L and M described above is shown in FIG.5. As seen from FIG. 5, in order to set optimum L and M, variousconditions need to be taken into consideration.

[0062] Values of L and M are notified from the controller 8, but atleast the value of M is finally determined by the base station, and thevalue of M determined by the base station is notified to the respectiveterminal stations. Additionally, here, the setting of M means not onlythe number of sub-carriers but also designation of the sub-carrier to beutilized.

[0063] On the other hand, the value of L may be determined by the basestation or the terminal station. FIG. 6 is a block diagram showing. theterminal station constitution in case that the terminal stationdetermines the value of L. In FIG. 6, constituting parts common to FIG.1 ate denoted with the same reference numerals. The terminal station ofFIG. 6 is constituted by adding a sub-carrier number determinationsection (sub-carrier number determination section) 15 to FIG. 1.

[0064] There are two techniques for the sub-carrier number determinationsection 15 of FIG. 6 to determine the value of L. In a first technique,only reception property of the packet is utilized. In this technique,the reception properties such as an error ratio property of thereception packet are measured, and L is increased with very satisfactoryreception properties. Conversely, when the reception properties aredeteriorated, L is decreased.

[0065] A second technique grasps the number of destination terminalstations of a multiple address packet in some method and by using theinformation, determines the value of L. As the method of grasping thenumber of destination terminal stations, a method of grasping the numberof destination terminal stations from destination addresses of themultiple address packet, a method of notifying the number of destinationterminal stations from the base station as information for determiningL, and the like are exemplified.

[0066] For a method of determining L and M (M is between L and N), forexample, when the number of terminal stations to perform the multicasttransmission of the packet is sufficiently small with respect to thetotal number N of sub-carriers constituting the OFDM symbol, M ispreferably decreased and L is increased. Thereby, both the erroneousdetection probability and the detection miss probability can be reduced.

[0067] Moreover, even when the number of terminal stations to performthe multicast transmission is large as compared with the total number Nof sub-carriers, but when it can be predicted that the number ofterminal stations to return the NAK signals is small (e.g., when packeterror ratio properties are very satisfactory), both the erroneousdetection probability and the detection miss probability can be reducedby decreasing M and increasing L.

[0068] On the other hand, when the number of destination terminalstations to perform the multicast transmission is very large and thepacket error ratio properties are insufficiently satisfactory, or whenit can be predicted that the number of terminal stations to return theNAK signals is large, by increasing M and decreasing L, the probabilityof selecting the same sub-carrier is reduced, and both the erroneousdetection probability and the detection miss probability can be reduced.

[0069] When L and M are determined in consideration of the number ofterminal stations to perform the multicast transmission andcommunication qualities such as the packet error ratio in this manner,both the erroneous detection probability and the detection missprobability can be reduced.

[0070] Moreover, as the method of determining L and M, there is also amethod of measuring a fluctuation of reception power for everysub-carrier in the NAK signal and feeding back the result. When the NAKsignals generated by superposing signal components to the samesub-carrier collide with one another, for a phase relation with the samephase, the power is doubled, and with the reverse phase, the powerbecomes zero.

[0071] Conversely, when no signal component is superposed to the samesub-carrier, the power fluctuation is influenced only by a propagationline, heat noise, or the like.

[0072] Therefore, another method can be considered which comprises firstdecreasing L and increasing M, and gradually increasing L and decreasingM until the power fluctuation increases or a sufficient NAK detectionprobability is obtained.

[0073]FIG. 7 is a block diagram showing a constitution of the basestation to perform the multicast transmission with respect to theterminal station shown in FIGS. 1 and 6 according to the firstembodiment. The base station of FIG. 7, as the reception systemconstitution, is provided with an RF section 22 for down-converting aradio frequency signal received by an antenna 21 to perform orthogonaldemodulation, an OFDM symbol detector 23 for performing FFT processingto an output of the RF section 22 to detect the OFDM symbol, a leveldetector 24 for detecting a reception level of a signal component foreach sub-carrier included in the OFDM symbol, a level judgment section(level judgment section) 25 for judging whether or not the receptionlevel of each signal component is a preset threshold T or more, a P/Sconverter 26 for performing a parallel/serial conversion of the OFDMsymbol, a demodulator 27 for demodulating the serially converted OFDMsymbol, an encoder 28 for performing an error detection based on ademodulation signal, and a controller 29 for receiving the demodulationsignal after the error detection.

[0074] The level judgment section 25 notifies the controller 29 toresend the packet corresponding to the NAK signal when the signalcomponent level is the threshold or more. Upon receiving thisnotification, the controller 29 resends the packet to the respectiveterminal stations via the transmission system of FIG. 7.

[0075] The base station of FIG. 7, as a transmission system (basestation resending section) constitution, is provided with: an encoder 30for encoding a transmission signal to generate a signal series, amodulator 31 for modulating each signal series generated by the encoder30, an S/P converter 32 for converting the modulation signal to aparallel signal, an OFDM symbol generator 33 for performing an IFFTprocessing to an output of the S/P converter 32 to generate the OFDMsymbol, and an RF section 34 for orthogonally modulating the OFDM symbolfor up-conversion to a radio frequency, and an output of the RF section34 is transmitted via the antenna 21.

[0076] Additionally, FIG. 7 shows an example in which the resendingpacket is accumulated in the controller 29, but the controller 29 doesnot necessarily have to perform packet buffering. For example, thesignal subjected to modulation by the modulator 31 or the OFDM symbolgenerated by the OFDM symbol generator may be buffered. In the bufferingby sections other than the controller 29, the resending request from thelevel judgment section 25 may be transmitted to a buffering place.

[0077] Moreover, the reception level detected by the level detector 24is not always transmitted to the level judgment section 25. When themulticast transmission is performed, the controller 29 grasps the timeslot to which the NAK signal is returned, and only the level of thesignal received in the time slot is therefore transmitted to the leveljudgment section 25.

[0078] Additionally, FIG. 7 shows only a minimum constitution todescribe the present invention, but similarly as the terminal station,when performing interleave or error correction, an interleaver, adeinterleaver, and the like are necessary.

[0079] Moreover, the level judgment section 25 does not need to performlevel detection of the reception signal in all N sub-carriersconstituting the OFDM symbol. As described above, the number M ofsub-carriers usable in the generation of the NAK signal not only meansthe number of sub-carriers, but also means the designation of thesub-carrier to be utilized. Therefore, the level judgment section 25 mayperform the level detection only of M sub-carriers notified from thecontroller 29. Thereby, the erroneous detection probability of the NAKsignal can be reduced.

[0080]FIG. 8 is a block diagram showing an internal constitution of thelevel judgment section 25 of FIG. 7. As shown in FIG. 8, the leveljudgment section 25 includes a selector 41 and a comparator 42. To thelevel judgment section 25 inputted are reception levels of allsub-carriers (N sub-carriers) detected by the level detector 24 of FIG.7. The selector 41 in the level judgment section 25 selects M signalsfrom N sub-carriers. The M signals are selected in accordance with theinstruction from the controller 29.

[0081] The M signals selected by the selector 41 are inputted to thecomparator 42. The comparator 42 judges whether or not the signal withthe reception level of the preset threshold T or more is present. Acomparison result by the comparator 42 is notified, for example, to thecontroller 29, and the controller 29 resends the packet subjected tobuffering. As described above, when the packet buffering is performed bysections other than the controller 29, the judgment result istransmitted to the buffering place.

[0082] As described above, in the first embodiment, when the multicasttransmission is performed to a plurality of terminal stations from thebase station in the OFDM system, and when an error is detected in thereception packet received by the terminal station, the NAK signalgenerated by using some of the sub-carriers constituting the OFDM symbolis returned to the base station, and both the erroneous detectionprobability and detection miss probability of the NAK signal cantherefore be reduced.

[0083] Moreover, since the number L of sub-carriers utilized forgenerating the NAK signal is determined in accordance with the number ofterminal stations, the error ratio property of the packet, and the like,a highly reliable multicast transmission is possible.

[0084] Furthermore, the base station having received the NAK signal fromthe terminal station resends the transmission packet to the terminalstation only when the reception level of the NAK signal exceeds thethreshold T, and therefore there is no possibility that the transmissionpacket is erroneously resent to the terminal station.

[0085] (Second Embodiment)

[0086] In a second embodiment, the base station determines the number Mof sub-carriers which can be utilized for generating the NAK signal.

[0087]FIG. 9 is a block diagram showing a constitution of the basestation of the second embodiment. In FIG. 9, constituting parts commonto FIG. 7 are denoted with the same reference numerals, and differentpoints will mainly be described hereinafter.

[0088] The base station of FIG. 9 is constituted by newly adding asub-carrier number determination section (sub-carrier numberdetermination section) 35 to FIG. 7.

[0089] The sub-carrier number determination section 35 determines atleast one of the number M of sub-carriers which can be utilized togenerate the NAK signal and the number L of sub-carriers actuallyutilized to generate the NAK signal.

[0090] When the terminal station is constituted as shown in FIG. 6, thesub-carrier number determination section 15 of FIG. 6 determines thenumber L of sub-carriers, and the sub-carrier number determinationsection 35 of FIG. 9 therefore determines only the number M ofsub-carriers. On the other hand, when there is no sub-carrier numberdetermination section 15 shown in FIG. 6 in the terminal station, thesub-carrier number determination section 35 of FIG. 9 determines boththe numbers L and M of sub-carriers.

[0091] As described above, in the second embodiment, since thesub-carrier number determination section 35 is disposed inside the basestation, the number L or M of sub-carriers can be changed in accordancewith the number of terminal stations, packet error property, and thelike, and both the erroneous detection probability and detection missprobability of the NAK signal can be reduced.

[0092] (Third Embodiment)

[0093] In a third embodiment, the threshold as a reference of detectionof the NAK signal is changed in accordance with the number L, M ofsub-carriers.

[0094]Fig. 10 is a block diagram showing a constitution of the basestation of the third embodiment. In FIG. 10, constituting parts commonto FIG. 7 are denoted with the same reference numerals, and differentrespects will mainly be described hereinafter.

[0095] The base station of FIG. 10 is constituted by newly adding athreshold determination section (threshold determination section) 36 toFIG. 7.

[0096] Generally, when the number M of sub-carriers able to be utilizedto generate the NAK signal is small, and the number L necessary forgenerating the NAK signal is large, collision of the NAK signals easilyoccurs, and the threshold is preferably increased. By increasing thethreshold, the erroneous detection probability is lowered, and with alarge L the detection miss probability is also lowered.

[0097] In this case, the threshold determination section 36 of FIG. 10determines the threshold T based on at least one of the numbers L, M ofsub-carriers notified from the controller 29, and notifies the leveljudgment section 25 of the value. The level judgment section 25 performsthe detection of the NAK signal based on the threshold T. Specifically,only when the reception level exceeds the threshold T, it is judged thatthe NAK signal is received.

[0098] As described above, in the third embodiment, since the value ofthe threshold T for judging the presence/absence of reception of the NAKsignal is set based on at least one of the numbers L, M of sub-carriers,the erroneous detection probability of erroneously judging that the NAKsignal is received can be lowered. Moreover, since the threshold T isset in relation to the number L of sub-carriers necessary for generatingthe NAK signal, the detection miss probability can also be lowered.

[0099] Moreover, the sub-carrier number determination section 35 of FIG.9 and the threshold determination section 36 of FIG. 10 may be added tothe base station constituted as shown in FIG. 7. Thereby, the number L,M of sub-carriers and the threshold T can simultaneously be controlled,and communication quality during the multicast transmission can furtherbe enhanced.

[0100] In the aforementioned first to third embodiments, an example hasbeen described in which the level detection section 24 in the basestation detects the reception signal level of the NAK signal for everysub-carrier based on the output of the OFDM symbol detector 3, but asanother example, the presence/absence of NAK signal may be judged basedon the detection result of the reception signal level of the OFDM signalwith a time waveform before the orthogonal demodulation in the RFsection 2. In this case, however, a reception signal level detectionrange has to be enlarged.

[0101] (Transmission Procedure of Multicast Transmission)

[0102]FIG. 11 shows a transmission procedure of the multicasttransmission in the aforementioned first to third embodiments. The basestation performs carrier sensing before transmitting the packet, judgesidleness for a first time interval called a distributed coordinationfunction interframe space (DIFS), then transmits the packet by themulticast transmission. This procedure is similar to that of the unicasttransmission defined by IEEE 802.11.

[0103] Each terminal station having received the packet from the basestation detects the error of the received packet, and generates the NAKsignal similarly as the first embodiment when the error is detected.Moreover, after receiving the multicast-transmitted packet, the terminalstation transmits the NAK signal after elapse of a second time intervalcalled a short interframe space (SIFS).

[0104] After transmitting the packet by the multicast transmission, thebase station waits for the elapse of the SIFS time before starting thedetection of the reception signal level. Moreover, after the packettransmission, the base station resends the previously transmitted packetwhen the reception signal level detected before the elapse of the DIFStime reaches the threshold T or more described in the first embodiment.If the reception signal level is less than the threshold T, theresending of the packet is not performed.

[0105] As described above, the present invention can be applied also tothe system of the CSMA base like the IEEE 802.11. Additionally, L, M,and the like are set similarly as the first embodiment.

What is claimed is:
 1. A radio base station for performingtransmission/reception of a packet to a plurality of terminal stationsin a multicarrier transmission system, the radio base stationcomprising: a level judgment section configured to judge whether or nota reception signal level of a resending request signal is a presetthreshold or more, said resending request signal being transmitted fromat least one of said plurality of terminal stations and obtained byusing L (M≧L, L is an integer) pieces of sub-carriers selected from atleast M (M≧1, M is an integer) pieces of sub-carriers; and a resendingsection configured to resend a multicast transmission packet to saidplurality of terminal stations only in case that it is judged that thereception signal level is said threshold or more.
 2. The radio basestation according to claim 1, wherein said multicarrier transmissionsystem is an orthogonal frequency division multiplexing (OFDM) system,and said level judgment section judge whether or not the receptionsignal level of the resending request signal is the preset threshold ormore in case that said resending request signal obtained by using the L(M≧L, L is an integer) pieces of sub-carriers selected from at least M(M≧1, M is an integer) pieces of sub-carriers included in an OFDM signalis received.
 3. A radio terminal station configured to performtransmission/reception of a packet with a base station in a multicarriertransmission system, said radio terminal station comprising: a receiverconfigured to receive a multicast transmission packet transmitted fromsaid base station; an error detector configured to detect whether or notthere is an error in said multicast transmission packet received by thereceiver; a sub-carrier selector configured to select L (M≧L, L is aninteger) pieces of sub-carriers from at least M (M≧1, M is an integer)pieces of sub-carriers included in a transmission signal as a resendingrequest signal to said multicast transmission packet; and terminalstation transmitter configured to transmit, to said base station, a saidresending request signal obtained by using said selected L sub-carriers.4. The radio terminal station according to claim 3, wherein saidmulticarrier transmission system is an orthogonal frequency divisionmultiplexing (OFDM) system, said sub-carrier selector select the L (M≧L,L is an integer) pieces of sub-carriers from at least M (M≧1, M is aninteger) pieces of sub-carriers included in an OFDM signal fortransmission which is the resending request signal to said multicasttransmission packet, and said terminal station transmitter transmit, tosaid base station, the OFDM signal obtained by using said selected Lsub-carriers as the resending request signal.